Is this right? "What we were discussing were the grounds on which he would make the motion."
A tough one. I had to look this up to be sure. Strange as it may seem, this sentence is correct. Pages 103-104 of the Grammar Desk Reference explain that the word "what" is not what the verb agrees with; rather, it's what comes later in the sentence. GDR gives these examples of correct sentences:
What is most essential is a clear explanation.
What are most essential are clear explanations.